Film acoustically-coupled transformer with increased common mode rejection

ABSTRACT

The film acoustically-coupled transformer (FACT) has a first and second decoupled stacked bulk acoustic resonators (DSBARs). Each DSBAR has a lower film bulk acoustic resonator (FBAR), an upper FBAR atop the lower FBAR, and an acoustic decoupler between them FBARs. Each FBAR has opposed planar electrodes and a piezoelectric element between the electrodes. A first electrical circuit interconnects the lowers FBAR of the first DSBAR and the second DSBAR. A second electrical circuit interconnects the upper FBARs of the first DSBAR and the second DSBAR. In at least one of the DSBARs, the acoustic decoupler and one electrode of the each of the lower FBAR and the upper FBAR adjacent the acoustic decoupler constitute a parasitic capacitor. The FACT additionally has an inductor electrically connected in parallel with the parasitic capacitor. The inductor increases the common-mode rejection ratio of the FACT.

RELATED APPLICATION

This disclosure is a Continuation-in-Part of U.S. patent applicationSer. No. 10/699,481 of John D. Larson III and Richard Ruby. Thisapplication is also related to U.S. patent application Ser. No. ______of John D. Larson III and Stephen Ellis entitled Pass Bandwidth Controlin Decoupled Stacked Bulk Acoustic Resonator Devices (Agilent Docket No.10040955-1) and U.S. patent application Ser. No. ______ of John D.Larson III, Richard Ruby and Stephen Ellis entitled FilmAcoustically-Coupled Transformer (Agilent Docket No. 10041306-1), bothfiled on the filing date of this disclosure. The above applications areassigned to the assignee of this application and the disclosures of theabove applications are incorporated into this application by reference.

BACKGROUND

Transformers are used in many types of electronic device to perform suchfunctions as transforming impedances, linking single-ended circuitrywith balanced circuitry or vice versa and providing electricalisolation. However, not all transformers have all of these properties.For example, an autotransformer does not provide electrical isolation.

Transformers operating at audio and radio frequencies up to VHF arecommonly built as coupled primary and secondary windings around a highpermeability core. Current in the windings generates a magnetic flux.The core contains the magnetic flux and increases the coupling betweenthe windings. A transformer operable in this frequency range can also berealized using an optical-coupler. An opto-coupler used in this mode isreferred to in the art as an opto-isolator.

In transformers based on coupled windings or opto-couplers, the inputelectrical signal is converted to a different form (i.e., a magneticflux or photons) that interacts with an appropriate transformingstructure (i.e., another winding or a light detector), and isre-constituted as an electrical signal at the output. For example, anopto-coupler converts an input electrical signal to photons using alight-emitting diode. The photons pass through an optical fiber or freespace that provides isolation. A photodiode illuminated by the photonsgenerates an output electrical signal from the photon stream. The outputelectrical signal is a replica of the input electrical signal.

At UHF and microwave frequencies, coil-based transformers becomeimpractical due to such factors as losses in the core, losses in thewindings, capacitance between the windings, and a difficulty to makethem small enough to prevent wavelength-related problems. Transformersfor such frequencies are based on quarter-wavelength transmission lines,e.g., Marchand type, series input/parallel output connected lines, etc.Transformers also exist that are based on micro-machined coupled coilssets and are small enough that wavelength effects are unimportant.However such transformers have issues with high insertion loss.

All the transformers just described for use at UHF and microwavefrequencies have dimensions that make them less desirable for use inmodern miniature, high-density applications such as cellular telephones.Such transformers also tend to be high in cost because they are notcapable of being manufactured by a batch process and because they areessentially an off-chip solution. Moreover, although such transformerstypically have a bandwidth that is acceptable for use in cellulartelephones, they typically have an insertion loss greater than 1 dB,which is too high.

Opto-couplers are not used at UHF and microwave frequencies due to thejunction capacitance of the input LED, non-linearities inherent in thephotodetector, limited power handling capability and insufficientisolation to give good common mode rejection.

Above-mentioned U.S. patent application Ser. No. 10/699,481, of whichthis disclosure is a continuation-in-part, discloses a filmacoustically-coupled transformer (FACT) based on decoupled stacked bulkacoustic resonators (DSBARs). A DSBAR is composed of a stacked pair offilm bulk acoustic resonators (FBARs) and an acoustic decoupler betweenthe FBARs. FIG. 1A schematically illustrates an embodiment 100 of suchFACT. FACT 100 has a first decoupled stacked bulk acoustic resonator(DSBAR) 106 and a second DSBAR 108 suspended above a cavity 104 in asubstrate 102. DSBAR 106 has a lower FBAR 110, an upper FBAR 120 stackedon lower FBAR 110, and an acoustic coupler 130 between them, and DSBAR108 has a lower FBAR 150, an upper FBAR 160 stacked on lower FBAR 150,and an acoustic coupler 170 between them. Each of the FBARs has opposedplanar electrodes and a piezoelectric element between the electrodes.For example, FBAR 110 has opposed planar electrodes 112 and 114 with apiezoelectric element 116 between them.

FACT 100 additionally has a first electrical circuit 141 interconnectingthe lower FBAR 110 of DSBAR 106 and the lower FBAR 150 of DSBAR 108 anda second electrical circuit 142 interconnecting the upper FBAR 120 ofDSBAR 106 and the upper FBAR 160 of DSBAR 108.

In the embodiment of the above-described FACT shown in FIG. 1A,electrical circuit 141 connects lower FBARs 110 and 150 in anti-paralleland to terminals 143 and 144 and electrical circuit 142 connects upperFBARs 120 and 160 in series between terminals 145 and 146. In theexample shown, electrical circuit 142 additionally has a center-tapterminal 147 connected to electrodes 122 and 162 of upper FBARs 120 and160, respectively. This embodiment has a 1:4 impedance transformationratio between electrical circuit 141 and electrical circuit 142 or a 4:1impedance transformation ratio between electrical circuit 142 andelectrical circuit 141.

In other embodiments, electrical circuit 141 electrically connects thelower FBARs 110 and 150 either in anti-parallel or in series, andelectrical circuit 142 electrically connects the upper FBARs either inanti-parallel or in series.

All embodiments of the above-described FACT are small in size, arecapable of linking single-ended circuitry with balanced circuitry orvice versa, and provide electrical isolation between primary andsecondary. The embodiments specifically described above are alsonominally electrically balanced.

The embodiment shown in FIG. 1A is of particular interest for a numberof applications. However, although this embodiment is nominallyelectrically balanced, its common mode rejection is less than manypotential applications require. The common-mode rejection of adifferential device is quantified by a common-mode rejection ratio(CMRR), which is the ratio of the differential-mode voltage gain to thecommon-mode voltage gain of the differential device.

One approach to increasing the common-mode rejection ratio is toincrease the thickness of the acoustic decoupler. However, increasingthe thickness of the acoustic decoupler causes the frequency response ofthe FACT to exhibit spurious artifacts caused by the ability of thethicker acoustic decoupler to support more than a single acoustic mode.Such spurious response artifacts are undesirable in many applications.

What is needed, therefore, is an FACT that has the advantages of theFACT described above, but that has an increased common mode rejectionratio and a smooth frequency response.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a film acoustically-coupledtransformer (FACT) that comprises a first decoupled stacked bulkacoustic resonator (DSBAR) and a second DSBAR. Each DSBAR comprises alower film bulk acoustic resonator (FBAR), an upper FBAR stacked atopthe lower FBAR, and an acoustic decoupler between the FBARs. Each FBARcomprises opposed planar electrodes and a piezoelectric element betweenthe electrodes. The FACT additionally comprises a first electricalcircuit connecting the lower FBAR of the first DSBAR to the lower FBARof the second DSBAR and a second electrical circuit connecting the upperFBAR of the first DSBAR to the upper FBARs of the second DSBAR. In atleast one of the DSBARs, the acoustic decoupler, one of the electrodesof the lower FBAR adjacent the acoustic decoupler and one of theelectrodes of the upper FBAR adjacent the acoustic decoupler constitutea parasitic capacitor. The FACT additionally comprises an inductorelectrically connected in parallel with the parasitic capacitor. Theinductor increases the common-mode rejection ratio of the FACT.

In a second aspect, the invention provides a film acoustically-coupledtransformer (FACT) having a pass band. The FACT comprises a firstdecoupled stacked bulk acoustic resonator (DSBAR) and a second DSBAR.Each DSBAR comprises a lower film bulk acoustic resonator (FBAR), anupper FBAR stacked atop the lower FBAR, and a piezoelectric elementbetween the electrodes. Each FBAR comprises opposed planar electrodesand a piezoelectric element between the electrodes. The FACTadditionally comprises a first electrical circuit connecting the lowerFBAR of the first DSBAR to the lower FBAR of the second DSBAR, and asecond electrical circuit connecting the upper FBAR of the first DSBARto the upper FBARs of the second DSBAR. In at least one of the DSBARs,the acoustic decoupler, one of the electrodes of the lower FBAR adjacentthe acoustic decoupler and one of the electrodes of the upper FBARadjacent the acoustic decoupler constitute a parasitic capacitor. TheFACT additionally comprises an element for forming, with the parasiticcapacitor, a parallel resonant circuit having a resonant frequency inthe pass-band of the FACT.

The inductor and the parasitic capacitor have a parallel resonance at afrequency in the pass-band of the FACT. Consequently, the path betweenthe first and second electrical circuits has a substantially higherimpedance in the pass-band than in an embodiment without the inductor.As a result, the common-mode rejection ratio is greater than in theconventional FACT that lacks the inductor.

In a final aspect, the invention provides a DSBAR device having aband-pass characteristic characterized by a center frequency. The DSBARdevice comprises a lower film bulk acoustic resonator (FBAR), an upperFBAR stacked on the lower FBAR, and an acoustic decoupler between theFBARs. Each FBAR comprises opposed planar electrodes and a piezoelectricelement between the electrodes. The acoustic decoupler structured toimpose a phase change nominally equal to π/2 on an acoustic signal equalin frequency to the center frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing of the electrical circuits of anembodiment of a 1:4 or 4:1 film acoustically-coupled transformer (FACT)in accordance with the prior art.

FIG. 1B is a schematic diagram showing the parasitic capacitor thatexists in the FACT shown in FIG. 1A when the center tap is grounded.

FIG. 1C is a schematic diagram showing the parasitic capacitors thatexist in the FACT shown in FIG. 1A when the center tap is floating.

FIG. 2A is a schematic diagram of an exemplary grounded center tapembodiment of a film acoustically-coupled transformer (FACT) with highcommon-mode rejection ratio (CMRR) in accordance with the invention.

FIG. 2B is a schematic diagram of an exemplary floating center tapembodiment of a FACT with high CMRR in accordance with the invention.

FIG. 3A is a schematic diagram of an exemplary grounded center tapembodiment of a FACT with high CMRR in accordance with the inventionthat provides DC isolation between primary and secondary.

FIG. 3B is a schematic diagram of an exemplary floating center tapembodiment of a FACT with high CMRR in accordance with the inventionthat provides DC isolation between primary and secondary.

FIGS. 4A, 4B and 4C are respectively a plan view and cross-sectionalviews along the section lines 4B-4B and 4C-4C in FIG. 4A of a FACTmodule with high CMRR that forms the basis of practical embodiments of aFACT in accordance with the invention.

FIG. 5 is a plan view of a first practical embodiment of a FACT withhigh CMRR in accordance with the invention.

FIG. 6 is a plan view of a second practical embodiment of a FACT withhigh CMRR in accordance with the invention.

FIG. 7 is a plan view of a third practical embodiment of a FACT withhigh CMRR in accordance with the invention that provides DC isolationbetween primary and secondary.

FIGS. 8A, 8B and 8C are respectively a plan view, a side view and across-sectional view along the section line 8C-8C in FIG. 8A of a fourthpractical embodiment of a FACT with high CMRR in accordance with theinvention.

FIGS. 8D and 8E are plan views of the substrates that constitute part ofthe FACT shown in FIGS. 8A-8C.

FIGS. 9A and 9B are respectively a plan view and a cross-sectional viewalong the section line 9B-9B in FIG. 9A of a fifth practical embodimentof a FACT with high CMRR in accordance with the invention.

FIGS. 10A and 10B are respectively a plan view and a cross-sectionalview along the section line 10B-10B in FIG. 10A of a sixth practicalembodiment of a FACT with high CMRR in accordance with the inventionthat provides DC isolation between primary and secondary.

FIGS. 11A-11H are plan views and FIGS. 11I-11P are cross-sectional viewsalong the section lines 11I-11I through 11P-11P in FIGS. 11A-11H,respectively, illustrating a process that may be used to fabricateembodiments of the FACT shown in FIGS. 10A-10B.

DETAILED DESCRIPTION

The inventors have discovered that, in embodiments of filmacoustically-coupled transformer (FACT) 100 described above withreference to FIG. 1A, a signal-frequency voltage difference can existbetween the electrodes on opposite sides of at least one of the acousticdecouplers 130 and 170 during normal operation. FIG. 1B shows FACT 100shown in FIG. 1A in a typical application in which terminal 144connected to electrodes 112 and 154 is grounded and a center tapterminal 147 connected to electrodes 122 and 162 is also grounded. Inthis application, a signal-frequency voltage difference exists betweenelectrodes 114 and 122 on opposite sides of acoustic decoupler 130. Whenapplied to the parasitic capacitor C_(P) composed of electrodes 114 and122 and acoustic decoupler 130, this voltage difference causes asignal-frequency current to flow between electrical circuit 141 andelectrical circuit 142. This current flow decreases the common moderejection of FACT 100. A capacitor symbol labelled C_(P) and depicted bybroken lines represents the parasitic capacitor C_(P) composed ofelectrodes 114 and 122 and acoustic decoupler 130. The capacitance ofthe parasitic capacitor is a maximum in embodiments in which thethickness of the acoustic decoupler is a minimum, i.e., the thickness ofthe acoustic decoupler is nominally equal to one quarter of thewavelength of an acoustic signal equal in frequency to the centerfrequency of the pass band of FACT 100. Such an acoustic decouplerimposes a phase change of π/2 on the acoustic signal.

In this disclosure, an element connected to a low impedance at thesignal frequency instead of to ground will be regarded as beinggrounded.

FIG. 1C shows FACT 100 in another exemplary application in whichelectrodes 112 and 154 are grounded, and electrical circuit 142 isfloating. In this application, a signal-frequency voltage differenceexists between electrodes 114 and 122 on opposite sides of acousticdecoupler 130, and a signal-frequency voltage difference additionallyexists between electrodes 154 and 162 on opposite sides of acousticdecoupler 170. When applied to the parasitic capacitor C_(P)′ composedof electrode 154, electrode 162 and acoustic decoupler 170, this voltagedifference causes an additional current to flow between electricalcircuit 141 and electrical circuit 142. This current flow furtherimpairs the common mode rejection ratio of FACT 100. A capacitor symbollabeled C_(P)′ and depicted by broken lines represents the parasiticcapacitor C_(P)′ provided by electrodes 154 and 162 and acousticdecoupler 170.

FIG. 2A is a schematic drawing showing an embodiment 200 of a filmacoustically-coupled transformer (FACT) in accordance with theinvention. FACT 200 is for use in an application similar to thatdescribed above with reference to FIG. 1B in which electrodes 112 and154 connected to terminal 144 are grounded and electrodes 122 and 162connected to center tap terminal 147 are also grounded. Elements of FACT200 that correspond to elements of FACT 100 described above withreference to FIG. 1B are indicated by the same reference numerals andwill not be described again here. In FACT 200, an inductor 180 isconnected between electrode 114 and electrode 122 on opposite sides ofacoustic decoupler 130. This connects inductor 180 in parallel withparasitic capacitor C_(P). Inductor 180 significantly increases thecommon-mode rejection ratio of FACT 200 relative to that of FACT 100 byreducing the current flow between electrical circuit 141 and electricalcircuit 142. Inductor 180 additionally improves the input match.

FACT 200 has a pass band. Inductor 180 and the parallel combination ofparasitic capacitor C_(P) and the capacitance C₀ between terminals 143and 144 form a parallel resonant circuit 182 having a resonant frequencyin the pass band. In one embodiment, the resonant frequency is equal tothe center frequency of the pass band of FACT 200. The impedance ofparallel resonant circuit 182 depends on a relationship between thesignal frequency and the resonant frequency of the resonant circuit, andis a maximum at the resonant frequency. At signal frequencies above andbelow the resonant frequency, the impedance of parallel resonant circuit182 is less than at the resonant frequency, but is substantially greaterthan that of parasitic capacitor C_(P) alone at all signal frequenciesin the pass band of FACT 200. Accordingly, the current that flowsbetween electrical circuit 141 and electrical circuit 142 throughparallel resonant circuit 182 is substantially less than that whichwould flow through parasitic capacitor C_(P) alone. Consequently, thecommon-mode rejection ratio of FACT 200 is greater than that of FACT 100shown in FIG. 1B.

FIG. 2B is a schematic drawing showing an embodiment 202 of a FACT inaccordance with the invention. FACT 202 is for use in an applicationsimilar to that described above with reference to FIG. 1C in whichelectrodes 112 and 124 are grounded and electrical circuit 142 isfloating. Elements of FACT 202 that correspond to elements of FACT 100described above with reference to FIG. 1B and of FACT 200 describedabove with reference to FIG. 2A are indicated by the same referencenumerals and will not be described again here. FACT 202 has an inductor180 connected between electrode 114 and electrode 122 on opposite sidesof acoustic decoupler 130 and an inductor 181 connected betweenelectrode 154 and electrode 162 on opposite sides of acoustic decoupler170. Inductors 180 and 181 significantly increase the common-moderejection ratio of FACT 202 relative to that of FACT 100 by reducing thecurrent flow between electrical circuit 141 and electrical circuit 142.Inductor 180 additionally improves the input match.

Inductor 180 and the parallel combination of parasitic capacitor C_(P)and inter-terminal capacitance C₀ form a parallel resonant circuit 182having a resonant frequency in the pass band of FACT 202. Inductor 181and parasitic capacitor C_(P)′ form a parallel resonant circuit 183having a resonant frequency in the pass band of FACT 202. In oneembodiment, parallel resonant circuits 182 and 183 have respectiveresonant frequencies equal to the center frequency of the pass band ofFACT 202. The impedance of parallel resonant circuits 182 and 183depends on a relationship between the signal frequency and the resonantfrequency of the respective resonant circuit, and is a maximum at theresonant frequency. At signal frequencies above and below the resonantfrequency, the impedance of the parallel resonant circuits 182 and 183is less than at the resonant frequency, but is substantially greaterthan that of parasitic capacitors C_(P) and C_(P)′ alone at all signalfrequencies in the pass band of FACT 202. Accordingly, the current thatflows between electrical circuit 141 and electrical circuit 142 throughparallel resonant circuits 182 and 183 is substantially less than thatwhich would flow through parasitic capacitors C_(P) and C_(P)′ alone.Consequently, the common-mode rejection ratio of FACT 202 is greaterthan that of FACT 100 in the application shown in FIG. 1C.

In FACT 200 shown in FIG. 2A, inductor 180 interconnects electricalcircuit 141 and electrical circuit 142 at DC. As a result, FACT 200 doesnot provide electrical isolation between electrical circuit 141 andelectrical circuit 142 at DC. FIG. 3A is a schematic drawing showing anembodiment 300 of a FACT in accordance with the invention thatadditionally provides electrical isolation between electrical circuits141 and 142 at DC voltages up to the breakdown voltage of an isolatingcapacitor 184 connected in series with inductor 180. FACT 300 is for usein an application similar to that described above with reference to FIG.1B in which electrodes 112 and 154 connected to terminal 144 aregrounded and electrodes 122 and 162 connected to center tap terminal 147are also grounded. Elements of the FACT 300 shown in FIG. 3A thatcorrespond to elements of FACT 200 shown in FIG. 2A are indicated usingthe same reference numerals and will not be described again here.

In FACT 300, isolating capacitor 184 and inductor 180 are connected inseries between electrode 114 and electrode 122 on opposite sides ofacoustic decoupler 130. As described above, inductor 180 and theparallel combination of parasitic capacitor C_(P) and inter-electrodecapacitance C₀ form a parallel resonant circuit 182 that reduces thecurrent flow between electrical circuit 141 and electrical circuit 142.Inductor 180 additionally forms a series resonant circuit with isolatingcapacitor 184. Typically, the capacitance of isolating capacitor 184 isat least four times that of the parallel combination of parasiticcapacitor C_(P) and inter-electrode capacitance C₀ so that the seriesresonant frequency of inductor 180 and isolating capacitor 184 is atleast one octave lower than the parallel resonant frequency of inductor180 and the parallel combination of parasitic capacitance C_(P) andinter-electrode capacitance C₀. This puts the series resonant frequencyoutside the pass band of FACT 300. As a result, isolating capacitor 184has a negligible effect on the frequency response of the parallelresonance in the pass band of FACT 300. The capacitance of isolatingcapacitor 184 may alternatively be less than that just described, but inthis case, the effect of isolating capacitor 184 on the frequencyresponse of the parallel resonance in the pass band of FACT 300 may beless than negligible.

FIG. 3B is a schematic drawing showing an embodiment 302 of a FACT inaccordance with the invention. FACT 302 is for use in an applicationsimilar to that described above with reference to FIG. 1C in whichelectrodes 112 and 154 are grounded and electrical circuit 142 isfloating. Elements of the FACT 302 shown in FIG. 3B that correspond toelements of FACT 202 shown in FIG. 2B are indicated using the samereference numerals and will not be described again here. FACT 302 hasinductor 180 and isolating capacitor 184 connected in series between theelectrode 114 and electrode 122 on opposite sides of acoustic decoupler130 and inductor 181 and an isolating capacitor 185 connected in seriesbetween electrode 154 and electrode 162 on opposite sides of acousticdecoupler 170.

Inductor 180 and isolating capacitor 184 connected in series reducecurrent flow between electrical circuit 141 and electrical circuit 142and isolate electrical circuit 141 from electrical circuit 142 at DC inthe manner described above. Inductor 181 and isolating capacitor 185connected in series reduce current flow between electrical circuit 141and electrical circuit 142 and isolate electrical circuit 141 fromelectrical circuit 142 at DC in a manner similar to that describedabove.

FIGS. 4A-4C are respectively a plan view and two cross-sectional viewsof an embodiment 400 of a film acoustically-coupled transformer (FACT)module that forms part of practical embodiments of a FACT with a highcommon-mode rejection ratio to be described below. Elements of FACTmodule 400 that correspond to elements of FACT 100 described above withreference to FIGS. 1A and 1B are indicated by the same reference numeraland will not be described again here.

FACT module 400 is composed of a substrate 102 and decoupled stackedbulk acoustic resonators (DSBARs) 106 and 108. Each DSBAR is composed ofa lower film bulk acoustic resonator (FBAR), an upper FBAR and anacoustic decoupler between the FBARs. FACT module 400 is additionallycomposed of an electrical circuit that connects the lower FBAR 110 ofDSBAR 106 to the lower FBAR 150 of DSBAR 108, and an electrical circuitthat connects the upper FBAR 120 of DSBAR 106 to the upper FBAR 160 ofDSBAR 108.

In DSBAR 106, lower FBAR 110 is composed of opposed planar electrodes112 and 114 and a piezoelectric element 116 between the electrodes, andupper FBAR 120 is composed of opposed planar electrodes 122 and 124 anda piezoelectric element 126 between the electrodes. In DSBAR 108, lowerFBAR 150 is composed of opposed planar electrodes 152 and 154 and apiezoelectric element 156 between the electrodes, and upper FBAR 160 iscomposed of opposed planar electrodes 162 and 164 and a piezoelectricelement 166 between the electrodes.

In FACT module 400, in DSBAR 106, acoustic decoupler 130 is locatedbetween lower FBAR 110 and upper FBAR 120; specifically, betweenelectrode 114 of lower FBAR 110 and electrode 122 of upper FBAR 120.Acoustic decoupler 130 controls the coupling of acoustic energy betweenFBARs 110 and 120. Acoustic decoupler 130 couples less acoustic energybetween the FBARs 110 and 120 than would be coupled if the FBARs were indirect contact with one another, Additionally, in DSBAR 108, acousticdecoupler 170 is located between FBARs 150 and 160; specifically,between electrode 154 of lower FBAR 150 and electrode 162 of upper FBAR160. Acoustic decoupler 170 controls the coupling of acoustic energybetween FBARs 150 and 160. Acoustic decoupler 170 couples less acousticenergy between the FBARs 150 and 160 than would be coupled if the FBARswere in direct contact with one another. The coupling of acoustic energydefined by acoustic decouplers 130 and 170 determines the bandwidth ofthe passband of FACT module 400.

In the example shown in FIGS. 4A-4C, acoustic decouplers 130 and 170 arerespective parts of an acoustic decoupling layer 131. Acousticdecoupling layer 131 is a layer of acoustic decoupling material. Oneimportant property of the acoustic decoupling material of acousticdecoupling layer 131 is an acoustic impedance significantly differentfrom than that of FBARs 110, 120, 150 and 160. Other importantproperties of the acoustic decoupling material are a high electricalresistivity and low dielectric permittivity to provide electricalisolation between the primary and secondary of the FACT.

Acoustic decoupling layer 131 has a nominal thickness t betweenelectrodes 114 and 122 and between electrodes 154 and 162 equal to anodd integral multiple of one quarter of the wavelength λ_(n) in theacoustic decoupling material of an acoustic signal equal in frequency tothe center frequency of the pass band of FACT module 400, i.e.,t≈(2m+1)λ_(n)/4, where m is an integer equal to or greater than zero.Such an acoustic decoupling layer imposes a phase change of an oddintegral multiple of π/2 radians on an acoustic signal having afrequency nominally equal to the center frequency of the pass band ofFACT module 400. An acoustic decoupling layer that differs from thenominal thickness by approximately 35 10% of λ_(n)/4 can alternativelybe used. A thickness tolerance outside this range can be used with somedegradation in performance, but the thickness of acoustic decouplinglayer 131 should differ significantly from an integral multiple ofλ_(n)/2.

Embodiments of FACT module 400 incorporating an embodiment of acousticdecoupling layer 131 in which the value of integer m is zero (t=λ_(n)/4)have a frequency response substantially closer to an ideal frequencyresponse than embodiments in which the acoustic decoupling layer has anominal thickness greater than λ_(n)/4 (m>0). Such an embodiment of theacoustic decoupling layer will be referred to as a minimum-thicknessacoustic decoupling layer. A minimum-thickness acoustic decoupling layerimposes a phase change of π/2 radians on an acoustic signal having afrequency nominally equal to the center frequency of the pass band ofFACT module 400. The frequency response of embodiments of the FACTmodule having a minimum-thickness acoustic decoupling layer lacks theabove-mentioned spurious response artifacts exhibited by embodiments inwhich the nominal thickness of the acoustic decoupling layer is greaterthan the minimum. As noted above, a smooth frequency response hashitherto been obtained at the expense of parasitic capacitor C_(P)having a substantially greater capacitance, and embodiments having asmooth frequency response have therefore typically had a low common-moderejection ratio. Embodiments of the FACT in accordance with theinvention use an inductor to reduce the effect of the high parasiticcapacitance resulting from a minimum-thickness acoustic decouplinglayer. Thus, embodiments of the FACT in accordance with the inventionhave both a high CMRR and the smooth frequency response provided by theminimum-thickness acoustic decoupling layer.

An inductor, or an inductor and a blocking capacitor in series, may beconnected between the electrodes located on opposite sides of theacoustic decoupler in any device, such as an acoustically-coupledtransformer or a band-pass filter, that incorporates one or more DSBARsto reduce effect of the parasitic capacitance between the constituentFBARs on the properties of the device. Such devices will be referred togenerically as DSBAR devices. Reducing the effect of the parasiticcapacitance allows the benefits of using a minimum-thickness acousticdecoupler to be obtained in any DSBAR device.

Many plastic materials have acoustic impedances in the range statedabove and can be applied in layers of uniform thickness in the thicknessranges stated above. Such plastic materials are therefore potentiallysuitable for use as the acoustic decoupling material of acousticdecoupling layer 131 that provides acoustic decouplers 130 and 170.However, the acoustic decoupling material must also be capable ofwithstanding the temperatures of the fabrication operations performedafter acoustic decoupling layer 131 has been deposited on electrodes 114and 154 to form acoustic decouplers 130 and 170. Electrodes 122, 124,162 and 164 and piezoelectric elements 126 and 166 are deposited bysputtering after acoustic decoupling layer 131 has been deposited.Temperatures as high as 300° C. are reached during these depositionprocesses. Thus, a plastic that remains stable at such temperatures isused as the acoustic decoupling material.

In one embodiment, a polyimide is used as the acoustic decouplingmaterial of layer 131. Polyimide is sold under the trademark Kapton® byE. I. du Pont de Nemours and Company. In such embodiment, acousticdecouplers 130 and 170 are composed of layer 131 of polyimide applied toelectrodes 114 and 154 by spin coating. Polyimide has an acousticimpedance of about 4 megarayleigh (Mrayl).

In another embodiment, a poly(para-xylylene) is used as the acousticdecoupling material of layer 131. In such embodiment, acousticdecouplers 130 and 170 are composed of layer 131 of poly(para-xylylene)applied to electrodes 114 and 154 by vacuum deposition.Poly(para-xylylene) is also known in the art as parylene. The dimerprecursor di-para-xylylene from which parylene is made and equipment forperforming vacuum deposition of layers of parylene are available frommany suppliers. Parylene has an acoustic impedance of about 2.8 Mrayl.

In another embodiment, the acoustic decoupling material of acousticdecoupling layer 131 is a crosslinked polyphenylene polymer. In suchembodiment, acoustic decoupling layer 131 is a layer of a crosslinkedpolyphenylene polymer Crosslinked polyphenylene polymers have beendeveloped as low dielectric constant dielectric materials for use inintegrated circuits and consequently remain stable at the hightemperatures to which acoustic decoupling layer 131 is subject duringthe subsequent fabrication of FBARs 120 and 160. The inventors havediscovered that crosslinked polyphenylene polymers additionally have acalculated acoustic impedance of about 2 Mrayl. This acoustic impedanceis in the range of acoustic impedances that provides FACT module 400with a useful pass bandwidth.

Precursor solutions containing various oligomers that polymerize to formrespective crosslinked polyphenylene polymers are sold by The DowChemical Company, Midland, Mich., under the trademark SiLK. Theprecursor solutions are applied by spin coating. The crosslinkedpolyphenylene polymer obtained from one of these precursor solutionsdesignated SiLK™ J, which additionally contains an adhesion promoter,has a calculated acoustic impedance of 2.1 Mrayl, i.e., about 2 Mrayl.

The oligomers that polymerize to form crosslinked polyphenylene polymersare prepared from biscyclopentadienone- and aromaticacetylene-containing monomers. Using such monomers forms solubleoligomers without the need for undue substitution. The precursorsolution contains a specific oligomer dissolved in gamma-butyrolactoneand cyclohexanone solvents. The percentage of the oligomer in theprecursor solution determines the layer thickness when the precursorsolution is spun on. After application, applying heat evaporates thesolvents, then cures the oligomer to form a cross-linked polymer. Thebiscyclopentadienones react with the acetylenes in a 4+2 cycloadditionreaction that forms a new aromatic ring. Further curing results in thecross-linked polyphenylene polymer. The above-described crosslinkedpolyphenylene polymers are disclosed by Godschalx et al. in U.S. Pat.No. 5,965,679, incorporated herein by reference. Additional practicaldetails are described by Martin et al., Development of Low-DielectricConstant Polymer for the Fabrication of Integrated Circuit Interconnect,12 ADVANCED MATERIALS, 1769 (2000), also incorporated by reference.Compared with polyimide, crosslinked polyphenylene polymers have a loweracoustic impedance, a lower acoustic attenuation and a lower dielectricconstant. Moreover, a spun-on layer of the precursor solution is capableof producing a high-quality film of the crosslinked polyphenylenepolymer with a thickness of the order of 200 nm, which is a typicalthickness of acoustic decoupling layer 131.

In another embodiment, acoustic decouplers 130 and 170 are composed ofacoustic decoupling layers (not shown) of acoustic decoupling materialshaving different acoustic impedances as described in the above-mentionedU.S. patent application Ser. No. ______ of John D. Larson III andStephen Ellis entitled Pass Bandwidth Control in Decoupled Stacked BulkAcoustic Resonator Devices. The acoustic impedances and thicknesses ofthe acoustic decoupling layers collectively define the acousticimpedance of, and phase change imposed by, acoustic decouplers 130 and170. The acoustic impedance of the acoustic decouplers in turn definesthe pass bandwidth of FACT module 400.

In an exemplary embodiment, the acoustic decouplers were composed of anacoustic decoupling layer of crosslinked polyphenylene polymer atop ofan acoustic decoupling layer of polyimide. Such acoustic decouplersprovide an embodiment of FACT module 400 with a pass bandwidthintermediate between the pass bandwidths of embodiments in which theacoustic decouplers are composed of single acoustic decoupling layer 131of polyimide or are composed of single acoustic decoupling layer 131 ofthe crosslinked polyphenylene polymer.

In an alternative embodiment, the acoustic decoupling material ofacoustic decoupling layer 131 has an acoustic impedance substantiallygreater than the materials of FBARs 110 and 120. No materials havingthis property are known at this time, but such materials may becomeavailable in future, or lower acoustic impedance FBAR materials maybecome available in future. The thickness of acoustic decoupling layer131 of such high acoustic impedance acoustic decoupling material is asdescribed above.

In another embodiment (not shown), acoustic decouplers 130 and 170 eachinclude a Bragg structure composed of a low acoustic impedance Braggelement sandwiched between high acoustic impedance Bragg elements. Thelow acoustic impedance Bragg element is a layer of a low acousticimpedance material whereas the high acoustic impedance Bragg elementsare each a layer of high acoustic impedance material. The acousticimpedances of the Bragg elements are characterized as “low” and “high”with respect to one another and additionally with respect to theacoustic impedance of the piezoelectric material of layers 116, 126, 156and 166. At least one of the Bragg elements additionally has a highelectrical resistivity and a low dielectric permittivity to provideelectrical isolation between input and output of FACT module 400.

Each of the layers constituting the Bragg elements has a nominalthickness equal to an odd integral multiple of one quarter of thewavelength in the material of the layer of an acoustic signal equal infrequency to the center frequency of FACT module 400. Layers that differfrom the nominal thickness by approximately ±10% of one quarter of thewavelength can alternatively be used. A thickness tolerance outside thisrange can be used with some degradation in performance, but thethickness of the layers should differ significantly from an integralmultiple of one-half of the wavelength.

In an embodiment, the low acoustic impedance Bragg element is a layer ofsilicon dioxide (SiO₂), which has an acoustic impedance of about 13Mrayl, and each of the high acoustic impedance Bragg elements is a layerof the same material as electrodes 114, 122, 154 and 162, e.g.,molybdenum, which has an acoustic impedance of about 63 Mrayl. Using thesame material for the high acoustic impedance Bragg elements and theelectrodes of FBARs 110, 120, 150 and 160 allows the high acousticimpedance Bragg elements additionally to serve as the electrodes of theFBARs adjacent the acoustic coupling elements.

DSBAR 106 and DSBAR 108 are located adjacent one another suspended overcavity 104 defined in a substrate 102. Suspending the DSBARs over acavity allows the stacked FBARs in each DSBAR to resonate mechanically.Other suspension schemes that allow the stacked FBARs to resonatemechanically are possible. For example, the DSBARs can be located over amismatched acoustic Bragg reflector (not shown) formed in or onsubstrate 102, as disclosed by Lakin in U.S. Pat. No. 6,107,721.

Referring additionally to FIG. 2A, a bonding pad 138 located on themajor surface of substrate 102 provides the signal terminal 143 ofelectrical circuit 141 of FACT module 400. A bonding pad 132 located onthe major surface of substrate 102 and a bonding pad 172 located on themajor surface of piezoelectric layer 117 that provides piezoelectricelements 116 and 156 collectively constitute the ground terminal 144 ofelectrical circuit 141. An interconnection pad 176 located on the majorsurface of the substrate, an electrical trace 177 extending fromelectrode 152 to interconnection pad 176, an interconnection pad 136 inelectrical contact with interconnection pad 176, an electrical trace 137extending from electrode 114 to interconnection pad 136, and anelectrical trace 139 extending from interconnection pad 176 to bondingpad 138 constitute the part of electrical circuit 141 that electricallyconnects electrode 114 of FBAR 110 to electrode 152 of FBAR 150 and tosignal terminal 143. Electrical trace 133 extending from electrode 112to bonding pad 132, electrical trace 167 extending from bonding pad 132to bonding pad 172 and electrical trace 173 extending from electrode 154to bonding pad 172 constitute the part of electrical circuit 141 thatelectrically connects electrode 112 of FBAR 110 to electrode 154 of FBAR150.

Bonding pad 134 and bonding pad 174 located on the major surface of thepiezoelectric layer 127 that provides piezoelectric elements 126 and 166constitute signal terminals 145 and 146 of electrical circuit 142.Bonding pad 178 located on the major surface of acoustic decouplinglayer 131 constitutes center-tap terminal 147 of electrical circuit 142.Bonding pads 163 and 168 located on the major surface of piezoelectriclayer 127 provide additional ground connections.

An electrical trace 171 that extends between electrode 122 and electrode162 over the surface of the acoustic decoupling layer 131 and anelectrical trace 179 that extends between electrical trace 171 andbonding pad 178 constitute the part of electrical circuit 142 thatconnects FBAR 120 and FBAR 160 in series and to center-tap terminal 147.An electrical trace 135 that extends between electrode 124 and bondingpad 134 and an electrical trace 175 that extends between electrode 154and bonding pad 174 constitute the part of electrical circuit 142 thatconnects FBAR 120 and FBAR 160 to signal terminals 145 and 146.Electrical trace 169 extends between bonding pad 163 and bonding pad 168that provide the ground terminals of electrical circuit 142. In thisembodiment electrical trace 169 additionally extends to bonding pad 178to connect center tap terminal 147 (FIG. 2A) to the ground of electricalcircuit 142.

Thousands of FACT modules similar to FACT module 400 are fabricated at atime by wafer-scale fabrication. Such wafer-scale fabrication makes theFACT modules inexpensive to fabricate. An exemplary fabrication processthat, with different masks, can be used to fabricate embodiments of FACTmodule 400 will be described below.

FIG. 5 is a plan view of a first practical embodiment 500 of a FACT withincreased CMRR in accordance with the invention. Elements of FACT 500that correspond to FACT module 400 shown in FIGS. 4A-4C are indicatedusing the same reference numerals and will not be described again here.

FACT 500 is composed of FACT module 400, a daughter board 511 andinductor 180 (FIG. 2A) embodied in the example shown as a surface-mountinductor 513. Defined in a conductive layer on the major surface 515 ofdaughter board 511 are bonding pads 521, 522, 523, 524, 525, 526, 527and 528, terminal pads 531, 532, 533, 534, 535, 536, 537, and 538 andattachment pads 541 and 543. Also defined in the conductive layer ofdaughter board 511 are a trace 551 that extends between bonding pad 521and terminal pad 531; a trace 552 that extends between bonding pad 522and terminal pad 532; a trace 553 that extends between bonding pad 523and terminal pad 533; a trace 554 that extends between bonding pad 524and terminal pad 534; a trace 555 that extends between bonding pad 525and terminal pad 535; a trace 556 that extends between bonding pad 526and terminal pad 536; a trace 557 that extends between bonding pad 527and terminal pad 537; and a trace 558 that extends between bonding pad528 and terminal pad 538.

Also defined in the conductive layer of daughter board 511 are a trace561 that extends between bonding pad 526 and attachment pad 541 and atrace 563 that extends between bonding pad 522 and attachment pad 543.

FACT module 400 is mounted on the major surface 515 of daughter board511 with bonding pads 172, 138, 132, 163, 134, 178, 174 and 168 oppositebonding pads 521, 522, 523, 524, 525, 526, 527 and 528, respectively.Bonding wires 571, 572, 573, 574, 575, 576, 577 and 578 extend between,and electrically connect, bonding pads 172, 138, 132, 163, 134, 178, 174and 168, respectively, of FACT module 400 and bonding pads 521, 522,523, 524, 525, 526, 527 and 528, respectively, of daughter board 511.

Alternatively, FACT module 400 is configured with terminal pads (notshown) located on the major surface (not shown) of substrate 102opposite major surface 103 in a manner similar to that described belowwith reference to FIGS. 8A-8C. The terminal pads are electricallyconnected by vias (not shown) extending through the substrate to bondingpads 172, 138, 132, 163, 134, 178, 174 and 168. Bonding pads 521, 522,523, 524, 525, 526, 527 and 529, respectively, are located on the majorsurface 515 of daughter board 511 in positions corresponding to thepositions of the terminal pads on FACT module 400. FACT module 400 isthen mounted on daughter board 511 with the terminal pads on the FACTmodule connected to the bonding pads on the daughter board using solderbumps or another suitable connection technique.

Surface-mount inductor 513 is mounted on attachment pads 541 and 543.Alternatively, a non surface-mount inductor may be electricallyconnected to attachment pads 541 and 543.

In FACT 500, one end of inductor 513 is electrically connected toelectrode 122 of FBAR 120 (FIG. 4B) by attachment pad 541, trace 561,bonding pad 526, bonding wire 576, bonding pad 178, trace 179 and partof trace 171 (FIG. 4A). Additionally, the other end of inductor 513 iselectrically connected to electrode 114 of FBAR 110 by attachment pad543, trace 563, bonding pad 522, bonding wire 572, bonding pad 138,trace 139, interconnection pads 176 and 136 and trace 137. Thus,inductor 513 is connected to electrodes 114 and 122 on opposite sides ofacoustic decoupler 130 in a manner similar to that shown in FIG. 2A.

In an example of FACT 500 structured for operation at a frequency ofabout 1.9 GHz, in which acoustic decouplers 130 and 170 had a nominalthickness equal to one quarter of the wavelength in the acousticdecoupling material of an acoustic signal equal in frequency to thecenter frequency of the pass band of the FACT, the parasitic capacitorC_(P) between electrodes 114 and 122 was about 1 pF, the capacitance C₀between input terminals 143 and 144 (FIG. 2A) was about 1.2 pF and theinductance of inductor 513 was about 3.2 nH.

FIG. 6 is a plan view of a second practical embodiment 502 of a FACTwith increased CMRR in accordance with the invention. Elements of FACT502 that correspond to FACT module 500 shown in FIG. 5 and FACT module400 shown in FIGS. 4A-4C are indicated using the same reference numeralsand will not be described again here.

In FACT 502, inductor 180 is embodied as a spiral trace 514 defined inthe conductive layer of daughter board 511. In this embodiment, daughterboard 511 is a multi-layer board and trace 565 is at a level below themajor surface 515 of the daughter board. Trace 565 is connected tospiral trace 514 and to trace 563 by vias 516. Alternatively, inductor180 may be embodied as a serpentine trace defined in the conductivelayer of daughter board 511. In this case, daughter board 511 need notbe a multi-layer board.

FIG. 7 is a plan view of a third practical embodiment 504 of a FACT withincreased CMRR in accordance with the invention. FACT 504 provides DCisolation between electrical circuits 141 and 142 (FIG. 3A). Elements ofFACT 504 that correspond to FACT module 500 shown in FIG. 5 and FACTmodule 400 shown in FIGS. 4A-4C are indicated using the same referencenumerals and will not be described again here.

FACT 504 is composed of FACT module 400, daughterboard 511, inductor 180(FIG. 3A) embodied in the example shown as surface-mount inductor 513,and isolating capacitor 184 (FIG. 3A) embodied in the example shown as asurface-mount capacitor 517. Additionally defined in a conductive layeron the major surface 515 of daughter board 511 are attachment pads 545and 547 and conductive traces 565 and 567. Conductive trace 565 extendsbetween attachment pad 543 and attachment pad 545 and conductive trace567 extends between attachment pad 547 and bonding pad 522.

Surface-mount inductor 513 is mounted on attachment pads 541 and 543 asdescribed above. Surface-mount capacitor 517 is mounted on attachmentpads 545 and 547. Alternatively, a non surface-mount inductor may beelectrically connected to attachment pads 541 and 543 and/or a nonsurface-mount isolating capacitor may be electrically connected toattachment pads 545 and 547. An inductor defined in the conductive layerof daughter board 511 similar to spiral trace 514 described above withreference to FIG. 6 may be substituted for attachment pads 541 and 543and surface-mount inductor 513.

In FACT 504, one end of inductor 513 is electrically connected toelectrode 122 of FBAR 120 (FIG. 4B) by attachment pad 541, trace 561,bonding pad 526, bonding wire 576, bonding pad 178, trace 179 (FIG. 4A)and part of trace 171 (FIG. 4A). The other end of inductor 513 iselectrically connected to one end of isolating capacitor 517 byattachment pad 543, trace 565 and attachment pad 545. The other end ofisolating capacitor 517 is connected to electrode 114 of FBAR 110 byattachment pad 547, trace 567, bonding pad 522, bonding wire 572,bonding pad 138, trace 139, interconnection pad 176, interconnection pad136 and trace 137. Thus, inductor 513 and isolating capacitor 517connected jn series are connected to electrodes 114 and 122 on oppositesides of acoustic decoupler 130 in a manner similar to that shown inFIG. 3A.

In an example similar to that described above with reference to FIG. 5in which the parasitic capacitor C_(P) was about 1 pF and inter-terminalcapacitance C₀ was about 1.2 pF, isolating capacitor 517 had acapacitance of about 8 pF and a breakdown voltage greater than themaximum DC voltage specified between electrical circuits 141 and 142(FIG. 3A).

Each of FACTs 500, 600 and 700 is used by mounting daughter board 511 onthe printed circuit board of a host electronic device (not shown), suchas a cellular telephone, and making electrical connections betweenterminal pads 531, 532, 533, 534, 535, 536, 537 and 538 andcorresponding pads on the printed circuit board. Many techniques areknown in the art for mounting a daughter board on a printed circuitboard and will therefore not be described here. Daughter board 511 mayalternatively be structured so that it can be mounted on the printedcircuit board of the host electronic device as a flip chip or usingsolder bumps.

FIGS. 8A, 8B and 8C are respectively a plan view, a side view and across-sectional view along the section line 8C-8C in FIG. 8A of a fourthpractical embodiment 600 of a FACT with high CMRR in accordance with theinvention. In FACT 600, DSBARs 106 and 108, electrical circuits 141 and142 and inductor 181 are enclosed in a hermetic enclosure of which thesubstrate of the FACT module forms part. FIGS. 8D and 8E arerespectively plan views of an embodiment 601 of FACT module 400 and anauxiliary substrate 611 that, together with an annular gasket 607, formFACT 600. Elements of FACT 600 that correspond to FACT module 400 shownin FIGS. 4A-4C are indicated using the same reference numerals and willnot be described again here.

FACT 600 is composed of embodiment 601 of FACT module 400 describedabove with reference to FIGS. 4A-4C, auxiliary substrate 611, annulargasket 607, and inductor 180 (FIG. 2A) embodied in the example shown assurface-mount inductor 613. FIG. 8D is a plan view of FACT module 601that forms part of FACT 600. FACT module 601 has a substrate 602 that isextended in the x- and y-directions relative to the substrate 102 of theembodiment of FACT module 400 described above with reference to FIGS.4A-4C. An annular pad 605 is located on the major surface 609 ofsubstrate 602 surrounding DSBARs 106 and 108 (FIG. 4A) and bonding pads172, 138, 132, 163, 134, 178, 174 and 168. Bonding pads 132 and 138 andinterconnection pad 176 are located on major surface 609. Annular gasket607 typically has a dimension in the z-direction greater than the sum ofthe z-direction dimensions of DSBAR 106 or DSBAR 108 and surface-mountinductor 613 and is located on annular pad 605.

A terminal pad is located on the major surface 615 of substrate 602opposite each of bonding pads 172, 138, 132, 163, 134, 178, 174 and 168.Major surface 615 is opposite major surface 609. A conductive viaextends through substrate 602 from each of the connection pads 132, 138,178, 168 and 172 to its respective attachment pad. The locations of vias621, 622, 623, 624, 625, 626, 627 and 628 are indicated by broken linesin FIG. 8D. The side view of FIG. 8B shows terminal pads 631 and 638located on major surface 615. The cross-sectional view of FIG. 8C showsterminal pads 632 and 636 located on major surface 615 opposite bondingpads 138 and 178, respectively, and electrically connected to bondingpads 138 and 178, respectively, by vias 622 and 626, respectively, thatextend through substrate 602.

Referring additionally to FIG. 8D, cylindrical interconnection posts 672and 676 are located on the surface of bonding pads 138 and 178,respectively. Interconnection posts 672 and 676 have a dimension in thez-direction greater than or equal to the dimension of gasket 607 in thez-direction.

FIG. 6E shows the major surface 617 of auxiliary substrate 611. Majorsurface 617 is opposite major surface 609 of substrate 602 when FACT 600is assembled. Located on major surface 617 are annular pad 619,connection pads 682 and 686, attachment pads 641 and 642 and electricaltraces 661 and 663. In an embodiment, annular pad 619, connection pads682 and 686, attachment pads 641 and 642 and electrical traces 661 and663 are defined in a conductive layer (not shown) located on majorsurface 617.

Annular pad 619 is similar in shape and dimensions to annular pad 605 onsubstrate 602 and engages with gasket 607 when FACT 600 is assembled.Connection pads 682 and 686 are similar in shape and dimensions tobonding pads 138 and 178 and are arranged on major surface 617 relativeto annular pad 619 such that they engage with interconnection posts 672and 676, respectively, when annular pad 619 is engaged with gasket 607.The positions of interconnection posts 672 and 672 and of gasket 607 atengagement are indicated by broken lines in FIG. 6E. Electrical trace661 extends from connection pad 686 to attachment pad 641 and electricaltrace 663 extends from connection pad 682 to attachment pad 643.

Surface-mount inductor 613 is mounted on attachment pads 641 and 643.Alternatively, a non surface-mount inductor may be electricallyconnected to attachment pads 641 and 643.

In FACT 600, one end of inductor 613 is electrically connected toelectrode 122 of FBAR 120 (FIG. 2B) by attachment pad 641, trace 661,connection pad 686, interconnection post 676, bonding pad 178, trace 179and part of trace 171 (FIG. 4A). Additionally, the other end of inductor613 is electrically connected to electrode 114 of FBAR 110 (FIG. 2B) byattachment pad 643, trace 663, connection pad 682, interconnection post672, bonding pad 138, trace 139, interconnection pads 176 and 136 andtrace 137. Thus, inductor 613 is connected to electrodes 114 and 122 onopposite sides of acoustic decoupler 130 in a manner similar to thatshown in FIG. 2A.

An inductor similar to spiral trace 514 described above with referenceto FIG. 6 may be defined in the conductive layer of auxiliary substrate611 and substituted for attachment pads 641 and 643 and surface-mountinductor 613. Additional attachment pads similar to attachment pads 545and 547 described above with reference to FIG. 7 may additionally bedefined in the conductive layer of auxiliary substrate 611. Electricaltraces additionally defined in the conductive layer electrically connecta surface-mount or other type of isolating capacitor mounted on theadditional attachment pads in series with the inductor betweenconnection pads 682 and 686 to provide DC isolation between electricalcircuits 141 and 142 in a manner similar to that described above withreference to FIGS. 3A, 3B and 7.

In an embodiment, gasket 607 is formed of a non-hermetic material coatedwith a layer of sealing material and interconnection posts 672 and 676are formed of a non-conductive material coated with a layer ofelectrically-conductive material as described in U.S. patent applicationSer. No. 10/890,343 of Larson III et al., assigned to the assignee ofthis disclosure and incorporated herein by reference. The same materialor different materials may be used as the non-hermetic material and thenon-conductive material. The same material or different materials may beused as the sealing material and the conductive material. In anotherembodiment, gasket 607 is formed of a material that bonds with siliconas described in U.S. Pat. No. 6,090,687 of Merchant et al., assigned tothe assignee of this disclosure and incorporated herein by reference.

FACT 600 is used by mounting it on the printed circuit board of a hostelectronic device (not shown) and attaching terminal pads 5631-638 tocorresponding pads on the printed circuit boards using solder bumps oranother suitable attachment process.

An exemplary process for fabricating FACT 600 will now be described.Although the fabrication of a single FACT will be described, theprocesses to be described are typically applied to wafers in whichthousands of devices identical to FACT 600 are formed.

FACT module 602 is fabricated using a process similar to that describedbelow with reference to FIGS. 11A-11P, but using different masks.Patterning one of the metal layers, typically the first metal layer,deposited in the course of the FACT module fabrication processadditionally defines annular pad 604 on the major surface 609 ofsubstrate 602.

Interconnection posts 672 and 676 and gasket 607 are formed on bondingpads 138 and 178 and annular pad 609, respectively, of substrate 602 bydepositing a layer of compliant material, such as polyimide on majorsurface 609. The layer of compliant material is patterned byphotolithography and a developing solvent to define interconnectionposts 672 and 676 and gasket 607. The interconnection posts and thegasket are then coated with a coating material. To coat interconnectionposts 672 and 676 and gasket 607, a seed layer (e.g., a layer oftitanium) is first sputtered onto the substrate and is removed from allbut interconnection posts 672 and 676 and gasket 607. Then,interconnection posts 672 and 676 and gasket 607 are electroplated witha relatively thick layer of an electrically-conductive material, such asgold. The coating makes gasket 607 and interconnection posts 672 and 676electrically conductive and additionally makes gasket 607 impervious togases such as air and water vapor.

Vias are formed in substrate 602 at locations underlying bonging pads172, 138, 132, 163, 134, 178, 174 and 168 are formed in substrate 602.Photolithography and anisotropic etching are used, or another suitablefabrication technique is used, to form holes that respectively extendthrough substrate 602 and, where present, the layers deposited onsubstrate 602, to the overlying bonding pads. The holes are then filledwith conductive material, such as copper or gold. A layer (not shown) ofelectrically-conducting material, such as gold, is then deposited on themajor surface 615 of substrate 602. The layer is patterned to define aterminal pad electrically connected to each of the vias and, hence, to arespective one of connection pads 172, 138, 132, 163, 134, 178, 174 and168. In an embodiment, gold is deposited by evaporation on major surface615. The gold is patterned to define the terminal pads. The thickness ofthe terminal pads is then increased by plating them with additionalgold. Terminal pads 631, 632, 636 and 638 are shown in FIGS. 8B and 8C.

A layer of electrically-conducting material (not shown) is deposited onthe major surface 617 of auxiliary substrate 611 by a suitabledeposition technique. Auxiliary substrate 611 is typically part of awafer of silicon, ceramic or another material. Ceramic has the advantageof having low electrical losses at microwave frequencies. Theelectrically-conductive material is typically gold, anotherelectrically-conductive material. The layer of electrically-conductingmaterial may be composed of two or more layers of different materials.Connection pads 682 and 686, attachment pads 641 and 643, electricaltraces 661 and 663 and annular pad 619 are defined in theelectrically-conducting layer using a suitable process such asphotolithography and etching or a lift-off process. The locations andshapes of connection pads 682 and 686 and the location and shape ofannular pad 619 on auxiliary substrate 611 respectively correspond tothe locations and cross-sectional shapes of interconnection posts 672and 676 and the location and shape of gasket 607 on substrate 602.However, the shape of connection pads 682 and 686 may differ from thecross-sectional shape of interconnection posts 672 and 676,respectively.

Surface-mount inductor 613 is mounted on attachment pads 641 and 643using a conventional surface-mount attachment technique. In embodimentssuch as that shown in FIG. 3A that have a capacitor in series withinductor 613, the capacitor is additionally mounted on its respectiveattachment pads. In embodiments in which the inductor 180 is embodied asa spiral or serpentine trace defined in the electrically-conductivelayer deposited on auxiliary substrate 611, no inductor mounting processis performed.

Auxiliary substrate 611 is inverted and is disposed opposite substrate602 with annular pad 619 and attachment pads 682 and 686 aligned withgasket 607 and interconnection posts 672 and 676, respectively.Auxiliary substrate 611 is then pressed against and bonded to substrate602. Pressing the substrates together puts interconnection posts 672 and676 in contact with connection pads 682 and 686, respectively, and putsgasket 607 in contact with annular pad 619. As the substrates arepressed together, the compliant material of the interconnection postsenables the interconnection posts to deform without fracturing orotherwise failing, and the compliant material of gasket 607 enables thegasket to deform without fracturing or otherwise failing. Typically,substrates 602 and 611 are bonded while being pressed together. Variousknown or future-developed bonding techniques may be used to bondsubstrates 602 and 611.

In one embodiment, thermal compression bonding is used. In suchembodiment, the electrically conductive material used to coatinterconnection posts 672 and 676 and gasket 607 is gold (Au). Beforesubstrates 602 and 611 are bonded, a layer of tin (Sn) is deposited onthe gold-coated interconnection posts and gasket. Substrates 602 and 611are then pressed together until interconnection posts 672 and 676 andgasket 607 make intimate contact with connection pads 682 and 686 andannular pad 619, respectively, and the assembly is heated until the goldand tin coating on the interconnection posts and gasket begins to melt.At this point, the coating material adheres to the material of annularpad 619 and connection pads 682 and 686. The assembly is then allowed tocool. The molten coating material solidifies as the assembly cools, andthe solidified material forms a bond between connection pads 682 and 686and interconnection posts 672 and 676, respectively, and between gasket607 and annular pad 619. The additional layer of tin on the gold-coatedinterconnection posts and gasket helps form a stronger bond during thethermal compression bonding.

The compliant materials of interconnection posts 672 and 676 and gasket607 ensure that interconnection posts 672 and 676 and gasket 607 onsubstrate 611 intimately contact connection pads 682 and 686 and annularpad 619 on auxiliary substrate 611. The compliant materials ofinterconnection posts 672 and 676 and gasket 607 allow interconnectionposts 672 and 676 and gasket 607 to deform until interconnection posts672 and 676 form a low-resistance electrical contact with connectionpads 682 and 686, respectively, and gasket 607 contacts annular pad 619along its entire periphery. For example, due to imperfections in thefabrication of interconnection posts 672 and 676 and gasket 607, it ispossible for gasket 607 to contact annular pad 619 before either or bothof interconnection posts 672 and 676 contact connection pads 682 and686, respectively. In this instance, gasket 607 deforms to allow thesubstrates 602 and 611 to be further pressed together until theinterconnection posts makes intimate contact with their respectivecontact pads. Similarly, either or both of interconnection posts 672 and676 or portions of gasket 607 may deform to allow the entire peripheryof the gasket to make intimate physical contact with annular pad 619.Intimate physical contact between the interconnection posts and theirrespective contact pads and between the gasket and annular pad 619during bonding helps to ensure that the interconnection posts provides areliable, low-impedance electrical conduction between electrodes 114 and112 and inductor 613 and that gasket 607 provides a reliable hermeticseal for the chamber bounded by substrates 602 and 611 and gasket 607.

The above-described process for assembling substrates 602 and 611 forform an hermetically-sealed chamber is described in more detailed in theabove-mentioned U.S. patent application Ser. No. 10/890,343, andadditionally in commonly-assigned U.S. patent application Ser. No.10/723,095 of Bai, which is also incorporated herein by reference.

FACT 600 is used by mounting it on the printed circuit board of a hostelectronic device using the terminal pads, such as terminal pads 631,632, 636 and 638 on the major surface 615 of substrate 602.

In a practical embodiment of FACT 202 shown in FIG. 2B or of FACT 302shown in FIG. 3B, an additional interconnection post (not shown) islocated on bonding pad 172 to provide an electrical connection toauxiliary substrate 611 from electrode 154. In such embodiment, aconnection pad (not shown) corresponding to the additionalinterconnection post and mounting pads for mounting an additionalsurface-mount inductor (and an optional additional isolation capacitor)are defined in the conductive layer of auxiliary substrate 611. Anadditional electrical trace (not shown) connects the other end of theadditional inductor to connection pad 686 directly or via the additionalisolation capacitor.

FIGS. 9A and 9B are respectively a plan view and a cross-sectional viewalong the section line 9B-9B in FIG. 9A of a fifth practical embodiment700 of a film acoustically-coupled transformer (FACT) with increasedCMRR in accordance with the invention. In FACT 700, an inductor isconnected between electrodes 114 and 122 in an arrangement similar tothat shown in FIG. 2A. The inductor is located on the substrate thatforms part of an embodiment of FACT module 400 described above withreference to FIGS. 4A-4C. Elements of FACT 700 that correspond to FACTmodule 400 described above with reference to FIGS. 4A-4C are indicatedusing the same reference numerals and will not be described again here.

FACT 700 is composed of an embodiment 701 of FACT module 400 describedabove with reference to FIGS. 4A-4C. In FACT module 701, substrate 702is extended in the −x-direction relative to the substrate 102 of theembodiment of FACT module 400. Piezoelectric layer 717 and acousticdecoupling layer 731 additionally extend over the extended portion ofsubstrate 702. Inductor 180 (FIG. 2A) is embodied as a spiral inductor713 defined in a conductive layer located on the surface of acousticdecoupling layer 731.

An electrical trace 763 extends in the −x-direction over the majorsurface of piezoelectric layer 717 from electrode 114 to aninterconnection pad 741. Acoustic decoupling layer 731 covers trace 763,but a window 733 defined in acoustic decoupling layer 731 providesaccess to interconnection pad 741. Spiral inductor 713 is structured asa spiral trace 714 located on the surface of acoustic decoupling layer731. An interconnection pad 743 that electrically contactsinterconnection pad 741 through window 733 is located at the inner endof spiral trace 714. An electrical trace 761 extends in the −x-directionover the surface of acoustic decoupling layer 731 from electrode 122 tothe outer end of spiral trace 714.

FIGS. 10A and 10B are respectively a plan view and a cross-sectionalview along the section line 10B-10B in FIG. 10A of a sixth practicalembodiment 704 of a film acoustically-coupled transformer (FACT) withincreased CMRR in accordance with the invention. In FACT 704, aninductor and a capacitor are connected in series between electrodes 114and 122 in an arrangement similar to that shown in FIG. 3A. Thecapacitor and the inductor are located on the substrate that forms partof an embodiment of FACT module 400 described above with reference toFIGS. 4A-4C. Elements of FACT 700 that correspond to FACT module 400described above with reference to FIGS. 4A-4C and FACT 700 describedabove with reference to FIGS. 9A and 9B are indicated using the samereference numerals and will not be described again here.

FACT 704 is composed of the above-described embodiment 701 of FACTmodule 400 in which substrate 702 is extended in the −x-direction andpiezoelectric layer 717 and acoustic decoupling layer 731 extend overthe extended portion of substrate 702. Inductor 180 (FIG. 3A) isembodied as a spiral inductor 713 defined in a conductive layer locatedon the surface of acoustic decoupling layer 731. Capacitor 184 (FIG. 3A)is embodied as a parallel-plate capacitor 715 having part of acousticdecoupling layer 731 as its dielectric.

One plate 718 of parallel-plate capacitor 715 is located on the majorsurface of piezoelectric layer 717. An electrical trace 763 extends inthe −x-direction over the major surface of piezoelectric layer 717 fromelectrode 114 to plate 718. An electrical trace 765 extends at about 45degrees to the x-direction over the major surface of piezoelectric layer717 from an interconnection pad 745 located outside plate 718 tointerconnection pad 741 located at the center of spiral inductor 713.Acoustic decoupling layer 731 covers trace 763, trace 765 and plate 718,but window 733 and a window 735 defined in acoustic decoupling layer 731provide access to interconnection pad 741 and interconnection pad 745,respectively.

The other plate 719 of capacitor 715 and spiral inductor 713 are locatedon the major surface of acoustic decoupling layer 731. Spiral inductor713 is structured as spiral trace 714 located on the major surface ofacoustic decoupling layer 731. An electrical trace 761 extends on overthe surface of acoustic decoupling layer 731 from electrode 122 to theouter end of spiral trace 714. Interconnection pad 743 that electricallycontacts interconnection pad 741 through window 733 is located at theinner end of spiral trace 714.

An electrical trace 767 extends in the y-direction over the majorsurface of acoustic decoupling layer 731 from plate 719 tointerconnection pad 747. Interconnection pad 747 electrically contactsinterconnection pad 745 through window 735 to complete the seriesconnection of inductor 713 and capacitor 715 between electrodes 114 and122.

In some embodiments, cavity 104 is extended so that it additionallyunderlies spiral inductor 713. This increases the separation between theinductor and the material of substrate 702, which reduces electricallosses.

In practical embodiments of the circuits shown in FIGS. 2B and 3B basedon the practical embodiments shown in FIGS. 9A, 9B, 10A and 10B,substrate 702, piezoelectric layer 717 and acoustic decoupling layer 731are additionally extended in the +x-direction (not shown). An additionalspiral inductor is located on the additional extension of the substratein the +x-direction and is connected between electrodes 154 and 162 asshown in FIG. 2B. Alternatively, an additional spiral inductor connectedin series with an additional parallel-plate capacitor are located on theadditional extension of the substrate in the +x-direction and areconnected between electrodes 154 and 162 as shown in FIG. 3B.

A process that can be used to fabricate FACT 704 described above withreference to FIGS. 10A and 10B will be described next with reference tothe plan views of FIGS. 11A-11H and the cross-sectional views of FIGS.11I-11P. The pass band of the embodiment of FACT 704 whose fabricationwill be described has a nominal center frequency of about 1.9 GHz.Embodiments for operation at other frequencies are similar in structureand fabrication but will have thicknesses and lateral dimensionsdifferent from those exemplified below. Moreover, with different masks,the process may be also be used to fabricate embodiments of the FACT 700described above with reference to FIGS. 9A and 9B and the variousembodiments of the FACT module 400 primarily described above withreference to FIGS. 4A-4C. Thousands of FACTs similar to FACT 704 arefabricated at a time by wafer-scale fabrication. Such wafer-scalefabrication makes the FACTs inexpensive to fabricate.

A wafer of single-crystal silicon is provided. A portion of the waferconstitutes, for each FACT being fabricated, a substrate correspondingto the substrate 702 of FACT 704. FIGS. 11A-11H and FIGS. 11-11Pillustrate, and the following description describes, the fabrication ofFACT 704 in and on a portion of the wafer. As FACT 704 is fabricated,the other FACTs on the wafer are similarly fabricated.

The portion of the wafer that constitutes the substrate 702 of FACT 704is selectively wet etched to form a cavity. A layer of fill material(not shown) is deposited on the surface of the wafer with a thicknesssufficient to fill each cavity. The surface of the wafer is thenplanarized, leaving each cavity filled with fill material. FIGS. 11A and11I show cavity 104 in substrate 702 filled with fill material 105.

In an embodiment, the fill material was phosphosilicate glass (PSG) andwas deposited using conventional low-pressure chemical vapor deposition(LPCVD). The fill material may alternatively be deposited by sputteringor by spin coating.

A first metal layer is deposited on the surface of substrate 702 andfill material 105. The first metal layer is patterned as shown in FIGS.11B and 11J to define electrode 112, electrode 152, bonding pad 132,bonding pad 138 and interconnection pad 176. The patterning also definesin the first metal layer electrical trace 133 extending betweenelectrode 112 and bonding pad 132, electrical trace 177 extendingbetween electrode 152 and interconnection pad 177, and electrical trace139 extending between interconnection pad 176 and bonding pad 138.

Electrode 112 and electrode 152 typically have an asymmetrical shape ina plane parallel to the major surface of the wafer. An asymmetricalshape minimizes lateral modes in FBAR 110 and FBAR 150 (FIG. 2A) ofwhich the electrodes form part. This is described in U.S. Pat. No.6,215,375 of Larson III et al., the disclosure of which is incorporatedinto this disclosure by reference. Electrode 112 and electrode 152 leavepart of the surface of fill material 105 exposed so that the fillmaterial can later be removed by etching, as will be described below.

Referring additionally to FIG. 2A, electrodes 114 and 154 are defined ina second metal layer, electrodes 122 and 162 are defined in a thirdmetal layer and electrodes 124 and 164 are defined in a fourth metallayer, as will be described below. The metal layers in which theelectrodes are define are patterned such that, in respective planesparallel to the major surface of the wafer, electrodes 112 and 114 ofFBAR 110 have the same shape, size, orientation and position, electrodes122 and 124 of FBAR 120 have the same shape, size, orientation andposition, electrodes 152 and 154 of FBAR 150 have the same shape, size,orientation and position and electrodes 162 and 164 of FBAR 160 have thesame shape, size, orientation and position. Typically, electrodes 114and 122 additionally have the same shape, size, orientation and positionand electrodes 154 and 162 additionally have the same shape, size,orientation and position.

In an embodiment, the material of each of the metal layers wasmolybdenum deposited by sputtering to a thickness of about 300 nm. Theelectrodes defined in each of the metal layers were pentagonal each withan area of about 12,000 square μm. Other electrode areas give othercharacteristic impedances. Other refractory metals such as tungsten,niobium and titanium may alternatively be used as the material of themetal layers. The metal layers may each alternatively comprise layers ofmore than one material. One factor to be considered in choosing thematerial of the electrodes of FACT 704 is the acoustic properties of theelectrode material: the acoustic properties of the material(s) of theremaining metal parts of FACT 704 are less important than otherproperties such as electrical conductivity. Thus, material(s) of theremaining metal parts of FACT 704 may be different from the material ofthe electrodes.

A layer of piezoelectric material is deposited and is patterned as shownin FIGS. 11C and 11K to define a piezoelectric layer 717 that providespiezoelectric element 116 of FBAR 110 and piezoelectric element 156 ofFBAR 150. Piezoelectric layer 717 extends over substrate 702 beyond theextent of cavity 104 to provide support for spiral inductor 713 andcapacitor 715. Piezoelectric layer 717 is patterned to expose part ofthe surface of fill material 105, bonding pads 132 and 138 andinterconnection pad 176. Piezoelectric layer 717 is additionallypatterned to define windows 119 that provide access to additional partsof the surface of the fill material.

In an embodiment, the piezoelectric material deposited to formpiezoelectric layer 717 and piezoelectric layer 727 described below wasaluminum nitride deposited by sputtering to a thickness of about 1.4 μm.The piezoelectric material was patterned by wet etching in potassiumhydroxide or by chlorine-based dry etching. Alternative materials forthe piezoelectric layers include zinc oxide, cadmium sulfide and poledferroelectric materials such as perovskite ferroelectric materials,including lead zirconium titanate, lead meta niobate and bariumtitanate.

A second metal layer is deposited on piezoelectric layer 717 and ispatterned as shown in FIGS. 11D and 11L to define electrode 114,electrode 154, plate 718 of capacitor 715 (FIG. 10A), bonding pad 172,interconnection pad 136 in electrical contact with interconnection pad176, and interconnection pads 741 and 745. The patterning additionallydefines in the second metal layer electrical trace 137 extending betweenelectrode 114 and interconnection pad 136, electrical trace 173extending between electrode 154 and bonding pad 172, electrical trace763 extending between electrode 114 and plate 718, electrical trace 765extending between interconnection pads 741 and 745, and electrical trace167 extending between bonding pads 132 and 172.

A layer of acoustic decoupling material is then deposited and ispatterned as shown in FIGS. 11E and 11M to define acoustic decouplinglayer 731 that provides acoustic decoupler 130 and acoustic decoupler170. Acoustic decoupling layer 731 extends over substrate 702 beyond theextent of cavity 104 to provide the dielectric of capacitor 715 andsupport for spiral inductor 713. Acoustic decoupling layer 731 ispatterned to cover at least electrode 114 and electrode 154, and toexpose part of the surface of fill material 105, bonding pads 132, 138and 172, and interconnection pads 136 and 176. Acoustic decoupling layer731 is additionally patterned to define windows 119 that provide accessto additional parts of the surface of the fill material, and to definewindows 733 and 735 that provide access to interconnection pads 741 and745, respectively.

In an embodiment, the acoustic decoupling material was polyimide with athickness of about 200 nm, i.e., one quarter of the center frequencywavelength in the polyimide. The polyimide was deposited to formacoustic decoupling layer 731 by spin coating, and was patterned byphotolithography. Polyimide is photosensitive so that no photoresist isneeded. As noted above, other plastic materials can be used as theacoustic decoupling material. The acoustic decoupling material can bedeposited by methods other than spin coating.

In an embodiment in which the acoustic decoupling material waspolyimide, after depositing and patterning the polyimide, the wafer wasbaked initially at a temperature of about 250° C. in air and finally ata temperature of about 415° C. in an inert atmosphere, such as anitrogen atmosphere, before further processing was performed. The bakeevaporates volatile constituents of the polyimide and prevents theevaporation of such volatile constituents during subsequent processingfrom causing separation of subsequently-deposited layers.

A third metal layer is deposited on acoustic decoupling layer 731 and ispatterned as shown in FIGS. 11F and 11N to define electrode 122,electrode 162, spiral trace 714 constituting spiral inductor 713, plate719 of capacitor 715 (FIG. 10A), bonding pad 178, interconnection pad743 at the inner end of spiral trace 714 in electrical contact withinterconnection pad 741 and interconnection pad 747 in electricalcontact with interconnection pad 745. The patterning also defines in thethird metal layer electrical trace 171 extending between electrode 122and electrode 162, electrical trace 179 extending between electricaltrace 171 and bonding pad 178, electrical trace 761 extending betweenelectrode 122 and the outer end of spiral trace 714, and electricaltrace 767 extending between plate 719 and interconnection pad 747.

A layer of piezoelectric material is deposited and is patterned as shownin FIGS. 11G and 11O to define piezoelectric layer 727 that providespiezoelectric element 126 of FBAR 120 and piezoelectric element 166 ofFBAR 150. Piezoelectric layer 727 is patterned to expose inductor 713,capacitor 715, bonding pads 132, 138, 178 and 172, interconnection pads136 and 176 and part of the surface of fill material 105. Piezoelectriclayer 727 is additionally patterned to define the windows 119 thatprovide access to additional parts of the surface of the fill material.

A fourth metal layer is deposited and is patterned as shown in FIGS. 11Hand 11P to define electrode 124, electrode 164, bonding pad 163, bondingpad 134, bonding pad 174 and bonding pad 168. The patterning alsodefines in the fourth metal layer electrical trace 135 extending fromelectrode 124 to bonding pad 134, electrical trace 175 extending fromelectrode 164 to bonding pad 174, and electrical trace 169 extendingfrom bonding pad 163 and bonding pad 168 to bonding pad 178.

The wafer is then isotropically wet etched to remove fill material 105from cavity 104. As noted above, portions of the surface of fillmaterial 105 remain exposed through, for example, windows 119. The etchprocess leaves FACT 704 suspended over cavity 104, as shown in FIGS. 10Aand 10B.

In an embodiment, the etchant used to remove fill material 105 wasdilute hydrofluoric acid.

A gold protective layer is deposited on the exposed surfaces of bondingpads 172, 138, 132, 163, 134, 178, 174 and 168.

The wafer is then divided into individual FACTs, including FACT 704.Each FACT is then mounted in a package and electrical connections aremade between bonding pads 172, 132, 163, 134, 178, 174 and 168 of theFACT and pads that are part of the package.

In one embodiment, FACT 704 is packaged in a hermetic package similar tothat described above with reference to FIGS. 8A-8E. However, componentsdifferent from inductor 180 and, optionally, capacitor 184 are mountedon the surface of the auxiliary substrate.

In another embodiment, the acoustic decoupling material of acousticdecoupling layer 731 is a crosslinked polyphenylene polymer After thesecond metal layer is patterned to define electrodes 114 and 154, asdescribed above with reference to FIGS. 11D and 11L, the precursorsolution for the crosslinked polyphenylene polymer is spun on in amanner similar to that described above with reference to FIGS. 11E and11M, but is not patterned. The formulation of the crosslinkedpolyphenylene polymer and the spin speed are selected so that thecrosslinked polyphenylene polymer forms a layer with a thickness ofabout 187 nm. This corresponds to one quarter of the wavelength λ_(n) inthe crosslinked polyphenylene polymer of an acoustic signal equal infrequency equal to the center frequency of the pass band of FACT 704.The wafer is then baked at a temperature in the range from about 385° C.to about 450° C. in an inert ambient, such as under vacuum or in anitrogen atmosphere, before further processing is performed. The bakefirst drives off the organic solvents from the precursor solution, andthen causes the oligomer to cross link as described above to form thecrosslinked polyphenylene polymer.

The third metal layer is then deposited on the layer of crosslinkedpolyphenylene polymer in a manner similar to that described above withreference to FIG. 11F, but is initially patterned in a manner similar tothat shown in FIG. 11E to define a hard mask that will be used topattern the layer of crosslinked polyphenylene polymer to defineacoustic decoupling layer 731. The initially-patterned third metal layerhas the same extent as acoustic decoupling layer 731 and has windows inthe following locations: over part of the surface of fill material 105,over bonding pads 132, 138 and 172, and in the intended locations ofwindows 119, 733 and 735 in acoustic decoupling layer 731.

The layer of crosslinked polyphenylene polymer is then patterned asshown in FIG. 11E with the initially-patterned third metal layer as ahard etch mask. The patterning defines the following features in thelayer of crosslinked polyphenylene polymer: the extent of acousticdecoupling layer 731, windows that provide access to part of the surfaceof fill material 105 and to bonding pads 132, 138 and 172, windows 733and 735 that provide access to interconnection pads 741 and 745,respectively, and windows 119 that provide access to additional parts ofthe surface of the fill material. The patterning is performed with anoxygen plasma etch.

The third metal layer is then re-patterned as shown in FIGS. 11F and 11Nto define electrode 122, electrode 162, spiral trace 714 constitutingspiral inductor 713, plate 719 of capacitor 715 (FIG. 10A), bonding pad178, interconnection pad 743 at the inner end of spiral trace 714 inelectrical contact with interconnection pad 741 and interconnection pad747 in electrical contact with interconnection pad 745. There-patterning also defines in the third metal layer electrical trace 171extending between electrode 122 and electrode 162, electrical trace 179extending between electrical trace 171 and bonding pad 178, electricaltrace 761 extending between electrode 122 and the outer end of spiraltrace 714, and electrical trace 767 extending between plate 719 andinterconnection pad 747.

Fabrication of the embodiment of FACT 704 with an acoustic decouplinglayer of crosslinked polyphenylene polymer is completed by performingthe processing described above with reference to FIGS. 11G, 11H, 11O and11P.

In an embodiment, the precursor solution for the crosslinkedpolyphenylene polymer was one sold by The Dow Chemical Company anddesignated SiLK™ J. Alternatively, the precursor solution may be anysuitable one of the precursor solutions sold by The Dow Chemical Companyunder the trademark SiLK. In certain embodiments, a layer of an adhesionpromoter was deposited before the precursor solution was spun on.Precursor solutions containing oligomers that, when cured, form acrosslinked polyphenylene polymer having an acoustic impedance of about2 Mrayl may be available from other suppliers now or in the future andmay also be used.

This disclosure describes the invention in detail using illustrativeembodiments. However, the invention defined by the appended claims isnot limited to the precise embodiments described.

1. A film acoustically-coupled transformer (FACT), comprising: a firstdecoupled stacked bulk acoustic resonator (DSBAR) and a second DSBAR,each DSBAR comprising: a lower film bulk acoustic resonator (FBAR) andan upper FBAR, the upper FBAR stacked atop the lower FBAR, each FBARcomprising opposed planar electrodes and a piezoelectric element betweenthe electrodes, and an acoustic decoupler between the FBARs; a firstelectrical circuit interconnecting the lower FBARs; and a secondelectrical circuit interconnecting the upper FBARs; in which: in atleast one of the DSBARs, the acoustic decoupler, one of the electrodesof the lower FBAR adjacent the acoustic decoupler and one of theelectrodes of the upper FBAR adjacent the acoustic decoupler constitutea parasitic capacitor; and the FACT additionally comprises an inductorelectrically connected in parallel with the parasitic capacitor.
 2. TheFACT of claim 1, in which: the FACT has a pass band; and the inductorand the parasitic capacitor constitute part of a parallel resonantcircuit having a resonant frequency in the pass band.
 3. The FACT ofclaim 1, in which: the FACT additionally comprises a substrate arrangedto support the DSBARs, the substrate having a major surface; and theinductor is located over the substrate.
 4. The FACT of claim 3, inwhich: the substrate comprises a cavity extending into the substratefrom the major surface thereof; and the inductor is suspended over thecavity.
 5. The FACT of claim 3, in which the inductor comprises: a layerof insulating material; and an inductive element over the layer ofinsulating material.
 6. The FACT of claim 5, in which at least part ofthe layer of insulating material and the piezoelectric element of one ofthe FBARs are portions of a common layer.
 7. The FACT of claim 5, inwhich at least part of the layer of insulating material and the acousticdecoupler are portions of a common layer.
 8. The FACT of claim 5, inwhich the layer of insulating material is located on the major surfaceof the substrate.
 9. The FACT of claim 5, in which at least part of theinductive element and one of the electrodes of one of the FBARs of eachof the DSBARs are portions of a common layer.
 10. The FACT of claim 1,in which: the FACT additionally comprises: a substrate supporting theDSBARs, and a daughter board comprising electrically-conducting traces;and the substrate and the inductor are mounted on the daughter boardelectrically interconnected by the conductive traces.
 11. The FACT ofclaim 1, in which: the FACT additionally comprises a package housing theDSBARs, the package comprising: a first substrate supporting the DSBARs,a second substrate disposed parallel to the first substrate, and agasket extending between the first substrate and the second substrate;and the inductor is mounted on the second substrate facing the firstsubstrate.
 12. The FACT of claim 11, in which the substrates and thegasket collectively define an hermetic chamber in which the DSBARs andthe inductor are located.
 13. The FACT of claim 11, in which: thepackage additionally comprises an interconnection post extending betweenthe first substrate and the second substrate; and the inductor iselectrically connected to one of the electrodes constituting theparasitic capacitor via the interconnection post.
 14. The FACT of claim1, in which: the FACT additionally comprises a package, comprising: afirst substrate supporting the DSBARs, a second substrate disposedparallel to the first substrate, and a gasket extending between thefirst substrate and the second substrate; and the inductor is defined ina metal layer located on a surface of the second substrate.
 15. The FACTof claim 14, in which: the package additionally comprises aninterconnection post extending between the first substrate and thesecond substrate; and the inductor is electrically connected to one ofthe electrodes of the parasitic capacitor via the interconnection post.16. The FACT of claim 1, additionally comprising an isolating capacitorelectrically connected in series with the inductor.
 17. The FACT ofclaim 16, in which: the inductor is part of a parallel resonant circuithaving a resonant frequency; and the inductor and the isolatingcapacitor have a series resonance at a frequency differing from theresonant frequency by more than one octave.
 18. The FACT of claim 17, inwhich: the isolating capacitor comprises a pair of parallel plates and adielectric between the parallel plates; one of the parallel plates andone of the electrodes of the parasitic capacitor are parts of a firstconductive common layer; the other of the parallel plates and the otherof the electrodes of the parasitic capacitor are parts of a secondcommon conductive layer; and the dielectric of the isolating capacitorand the acoustic coupler are parts of a third common layer.
 19. The FACTof claim 1, in which: the FACT has a pass band having a centerfrequency; and each of the acoustic decouplers comprises a layer ofacoustic decoupling material having a thickness nominally equal to onequarter of the wavelength in the acoustic decoupling material of anacoustic signal equal in frequency to the center frequency.
 20. The FACTof claim 1, in which the acoustic decoupling material comprises one ofpolyimide, paralene and crosslinked polyphenylene polymer.
 21. A filmacoustically-coupled transformer (FACT) having a pass band, the FACTcomprising: a first decoupled stacked bulk acoustic resonator (DSBAR)and a second DSBAR, each DSBAR comprising: a lower film bulk acousticresonator (FBAR) and an upper FBAR, the upper FBAR stacked atop thelower FBAR, each FBAR comprising opposed planar electrodes and apiezoelectric element between the electrodes, and an acoustic decouplerbetween the FBARs, a first electrical circuit interconnecting the lowerFBARs; and a second electrical circuit interconnecting the upper FBARs;in which: in at least one of the DSBARs, the acoustic decoupler, one ofthe electrodes of the lower FBAR adjacent the acoustic decoupler and oneof the electrodes of the upper FBAR adjacent the acoustic decouplerconstitute a parasitic capacitor; and the FACT additionally comprisesmeans for forming with the parasitic capacitor a parallel resonantcircuit having a resonant frequency in the pass band.
 22. The FACT ofclaim 21, in which the means for forming comprises an inductor and anelectrical circuit electrically connecting the parasitic capacitor tothe inductor.
 23. A DSBAR device having a band-pass characteristiccharacterized by a center frequency, the DSBAR device comprising: alower film bulk acoustic resonator (FBAR) and an upper FBAR stacked onthe lower FBAR, each FBAR comprising opposed planar electrodes and apiezoelectric element between the electrodes; and an acoustic decouplerbetween the FBARs, the acoustic decoupler structured to impose a phasechange nominally equal to π/2 on an acoustic signal equal in frequencyto the center frequency.
 24. The DSBAR device of claim 23, in which: theacoustic decoupler comprises no more than one acoustic decoupling layerof acoustic decoupling material; and the acoustic decoupling layer has athickness nominally equal to one quarter of the wavelength of theacoustic signal in the acoustic decoupling material.
 25. The DSBARdevice of claim 23, in which the acoustic decoupler comprises acousticdecoupling layers of acoustic decoupling materials having differentacoustic impedances.
 26. The DSBAR device of claim 23, in which: theacoustic decoupler and adjacent ones of the electrodes of the lower FBARand the electrodes of the upper FBAR constitute a parasitic capacitor;and the DSBAR device additionally comprises an inductor electricallyconnected in parallel with the parasitic capacitor.